Terahertz (T-ray or THz) radiation lies in the far-infrared (FIR) region of the electromagnetic spectrum. More specifically, the terahertz band of the electromagnetic spectrum exists between the mid-infrared band and the microwave band. Loosely defined, the terahertz band encompasses that part of the frequency spectrum that includes the frequencies ranging from about 0.3–10.0 THz or, equivalently, the wavelengths ranging from about 1.0 to 0.03 millimeters.
THz radiation has some unique features. For example, THz waves easily penetrate most non-metallic objects like paper, cardboard, plastics, and moderate thickness of many dielectrics, yet are absorbed by polar materials and liquids. Carriers in semiconductors show strong dielectric response in the terahertz region of the spectrum; metals are substantially opaque to THz radiation. Polar gases such as water vapor, ammonia, hydrogen chloride, and the like have strong and very characteristic absorption lines in the terahertz region. Consequently, the THz spectral range is becoming increasingly important for applications such as remote sensing of gases, quality control of plastic and composite materials, and moisture analysis. In addition, the terahertz frequency range has been of considerable interest in spectroscopy. For example, the electronic properties of semiconductors and metals are greatly influenced by bound states (e.g., excitons and Cooper pairs) whose energies are resonant with THz photons.
More recently, significant applications in optical imaging have become practical. THz radiation imaging shows promise in a variety of analytical imaging applications, such as chemical mapping, and a host of commercial applications such as safe package inspection, industrial quality and process control, food inspection, biology, and medicine. Promising applications also include contamination measurements, chemical analysis, wafer characterization, remote sensing, and environmental sensing.
Within the next decade, x-ray imaging systems will be replaced by imaging systems using terahertz frequency sources and detectors in areas such as medical, security, and quality control applications. T-rays can penetrate most solid substance like x-rays. In contrast to x-rays, however, T-rays are non-ionizing and, therefore, are non-lethal and safer for imaging applications. Further, T-ray systems produce true high resolution images rather than shadowy images produced by x-ray systems.
A heavy demand for terahertz technology also exists in the communications industry. The anticipated development of components necessary for a terahertz frequency heterodyne receiver will result in a dramatic increase in the available bandwidth in wavelength-division-multiplexed communications networks. In summary, there is a growing appreciation for the many potential commercial applications in which terahertz spectroscopy and imaging might be exploited.
Despite its potential, the use of THz electromagnetic signals for such applications as spectroscopy and imaging has been hindered by a lack of suitable tools—including generators for the THz radiation. For example, swept-frequency synthesizers for millimeter— and submillimeter-waves are limited to below roughly 100 GHz, with higher frequencies previously available only through the use of discrete frequency sources. Fourier transform infrared spectroscopy (FTIR), on the other hand, remains hampered by the lack of brightness of incoherent sources. In addition, FTIR methods are not useful if the real and imaginary part of response functions must be measured at each frequency. Finally, real-time imaging using the THz range of the electromagnetic spectrum has not been possible so far due to the poor sensitivity of detectors in this frequency range.
Focusing on generators for THz radiation, although advances in these devices have been significant, generators still operate with a low conversion efficiency. Such inefficiency limits the dynamic range, signal-to-noise, and detectability of the terahertz beams. Continued advances in the technology of terahertz generators requires the conversion process to have improved efficiency. Moreover, THz rays have relatively low average power which renders such radiation unsuitable for some applications.
Various patents define the field of THz radiation generation. For the purpose of providing additional background into this field, a sample of such patents has been collected. Five of the patents are summarized below.
1. U.S. Pat. No. 5,937,118
The '118 patent issued to Komori is directed to a quantum synthesizer, a THz electromagnetic wave generation device, an optical modulation device, and an electron wave modulation device. The electromagnetic wave generation device includes an ultrashort light pulse for a phase-locked multi-wavelength light (reference number 2 in the figures of the '118 patent) that is made incident on a coded excitation light generation portion 3. The phases and amplitudes of the incident light are controlled responsive to frequencies to obtain coded excitation light. The coded excitation light is imputed into a quantum synthesizer 10 having a quantum synthesis portion 1. By this procedure, a THz electromagnetic wave 4 having an arbitrary frequency is generated.
Also included in the disclosed device are quantum wires 19. FIGS. 15A and 15B of the '118 patent illustrate a method of fabricating a quantum wire structure. FIG. 15A shows a single-layer structure; FIG. 15B, a multi-layer structure. First, V-grooves 28 are formed in a semiconductor substrate 21, and then quantum well layers 29 and barrier layers 30 are alternately grown in the V-grooves 28 to form multi-coupled quantum wires. More specifically, V-grooves 28 are formed in a gallium arsenide (GaAs) substrate 21, and a GaAs buffer layer and an AlGaAs buffer layer are formed on the substrate. Further, “n” number each of GaAs quantum wire (quantum well) layers 29 and AlGaAs barrier layers 30 are alternately grown. Thus, there is produced a multi-coupled quantum wire structure comprising the n number of very fine quantum wires 5 nm thick and 30 nm wide (effective width 15 nm) and barrier layers 2 nm thick formed at the bottom of the V-groove 28.
FIG. 20A of the '118 patent shows an example of the band structure of a quantum synthesizer (an excited electron wave synthesis portion corresponding to the numeral 1 of FIG. 6) for exciting the quantum synthesis portion of the invention by optical excitation to generate a THz electromagnetic wave. Ultrashort pulsed light (femtosecond light) with phases and amplitudes controlled for the respective frequencies is formed at an excitation light coding portion (corresponding to the numeral 3 in FIG. 6). Such ultrashort pulsed light is obtained by a commercially available laser, e.g., a Ti-sapphire laser.
2. U.S. Pat. No. 5,729,017
The '017 patent issued to Brener et al. is directed to terahertz generators and detectors. The disclosed system includes a semiconductor substrate 11 that has strip electrodes 12 and 13 interconnected with a DC bias 14. The dipole members 15 and 16 form an electrode gap 17 which is the active site of the device. A laser spot 18 from the pump laser is incident on a portion or all of the gap. The pump source is typically a femtosecond pulse laser operating at a wavelength of 500 nm to 2,000 nm and a pulse duration of 10 picoseconds or less. The generated THz signal radiates in all directions, but a large fraction is emitted into the substrate 11 and is collected from the back side of substrate 11. The disclosed invention focuses on the gap geometry, as defined by various shapes (see FIGS. 2–5 of the '017 patent) given to the dipole members.
3. U.S. Pat. No. 6,479,822
The '822 patent issued to Nelson et al. is directed to a system and method for making spectroscopic and metrology measurements of a sample using terahertz frequency electromagnetic radiation. The disclosed system includes an optical light source 20 and a non-centrosymmetric crystal 30. The optical light source 20 directs a pair of excitation beams 22 to spatially overlap and interfere with one another to form a grating pattern in the non-centrosymmetric crystal 30. The coherent bandwidth of the excitation beams 22 is sufficient to excite a polariton 32 in crystal 30 corresponding to the wave vector of the grating pattern. Polariton 32 propagates to the edge of crystal 30 where its electromagnetic component then couples into sample 50 as electromagnetic terahertz radiation 52. The properties of the sample 50 can alter the characteristics of the terahertz radiation 52 which, following its interaction with sample 50, couples into a second non-centrosymmetric crystal 60 as another polariton 62. Light source 20 directs a temporally delayed probe beam 27 to interact with polariton 62 and produce a signal beam 66, whose intensity is measured by a detector 70. A computer 80 is coupled to the detector 70 and light source 20 to analyze the signal measured by the detector with respect to the properties of the excitation beams 22 and the probe beam 27.
4. U.S. Pat. No. 6,605,808
The '808 patent issued to Mickan et al. is directed to a diagnostic apparatus using terahertz radiation. The disclosed system includes a stainless steel enclosure 1 that contains a generator and a detector for terahertz radiation. The generator comprises a femtosecond laser 2 producing laser radiation 3 within the range of 800–900 nm and in pulse widths of less than 100 fs, and a zinc telluride terahertz generating crystal 4. Under the influence of the laser radiation, the crystal 4 produces terahertz radiation 5. The laser 2 is outside the enclosure 1 and its radiation is directed through a laser window 6. The generator directs terahertz radiation onto a target 12. Reflected terahertz radiation is returned through a reflection receiving window into the enclosure 1 and to the detector 13. A modified atmosphere is provided within the enclosure to permit ready transmission of the terahertz radiation.
5. U.S. Pat. No. 6,075,640
The '640 patent issued to Nelson is directed to signal processing by optically manipulating polaritons. The disclosed system includes a signal source 14 that further includes a waveguide 22 that delivers a high bandwidth terahertz electromagnetic radiation 26 to signal processing material 12 as an input signal. The terahertz radiation can be generated by any of a number of ways known in the art. For example, terahertz radiation can be produced using ultrafast optical signals from a femtosecond laser to modulate electromagnetic radiation emitted from gallium arsenide (GaAs) quantum well structures.
To overcome the shortcomings of conventional generators of THz radiation, a new THz radiation emitter is provided. An object of the present invention is to provide an improved THz emitter having an efficient dipole geometry and minimal Fresnel reflection. A related object is to modify the surface structure of a semiconductor as the THz emitter. Another object is to maximize the power of the THz radiation emitted. Still another object is to improve the conversion efficiency of the THz radiation emitter.